Engineering Failure Analysis 15 (2008) 83–89

www.elsevier.com/locate/engfailanal

Is 7206 ISO standard enough to prove the endurance offemoral components of hip prostheses?

Jesus Chao

Centro Nacional de Investigaciones Metalurgicas (CSIC), Avda. Gregorio del Amo, 8, 28040 Madrid, Spain

Received 20 November 2006; accepted 20 November 2006Available online 16 January 2007

Abstract

The design of a cementless femoral stem that fractured at the neck–shoulder junction was analysed with reference toparts 4 and 8 of ISO standard 7206. The stresses in the section where fracturing occurred were calculated assuming the2300 N loading specified in ISO 7206-8. The results show that the prosthesis met ISO standard requirements regardingfatigue behaviour. However, this feature does not occur when a load of 4000 N, corresponding to a patient with a bodyweight of 1000 N, is applied. It is therefore suggested that patient body weight should be taken into account when design-ing and choosing the appropriate stem size.� 2006 Elsevier Ltd. All rights reserved.

Keywords: 7206 ISO standard; Fatigue design; Cementless Hip prosthesis; Ti6Al4V alloy

1. Introduction

Total hip arthroplasty (THA) has proven to be a successful method for the relief of pain and the restorationof normal daily activities in patients with hip disablement. The long-term success of this operation depends ona variety of factors, such as the surgical technique employed, the positioning and alignment of the stem, thestability of the intramedullary fixation, the patient weight, and the implant design. A recent study on a pop-ulation of 7695 hip prostheses (implanted during the period 1990–2000) [1] revealed that 24.8% of these pros-theses had been revised, i.e. one or both of the prosthesis components had required replacement. The studiedprosthesis population consisted of: 17% cemented, 54% cementless, 28.6% with cementless cup and cementedstem, and 0.4% with cemented cup and cementless stem. Stem fracture was the reason for 1.7% of the implantrevisions, while the most frequently observed cause (86.5%) was loosening. On the basis of these figures, it maybe thought that mechanical failure of the femoral component is a rare complication in THA. However, theauthor considers that this is not an unimportant problem, above all when the retrieval of the stem is extremelydifficult and necessitates deliberate damage to the femur [2,3]. Case reports of in vivo fracture of cementless hipprosthesis have attributed this event to: a weak junction between the madreporic corrugations and the smooth

1350-6307/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2006.11.017

E-mail address: jchao@cenim.csic.es

84 J. Chao / Engineering Failure Analysis 15 (2008) 83–89

plate [4]; the smelting technology used and cracking of the porous layer [5]; the narrowness of the antero-pos-terior dimensions of the proximal region of the stem [2]; a galvanic corrosion–fatigue process at the taperedinterface between the neck and the head of modular femoral components [6,7]; a lack of support in the prox-imal region of the stem and the distal stem fixation [8]; and the machining induced residual stresses on thesurface of the stress concentration at the neck–shoulder junction [3]. The second last failure mode [8] maybe justifiable because the stem fracture is preceded of loosening of the proximal part of the implant [9],and the loosening issue is still a topic subjected to investigation. The other failure modes are the result of mis-takes in the design and/or control method of the femoral component.

In a previous paper, it was analysed the fractographic features and metallurgical factors of the in vivo fati-gue failure at the neck–shoulder junction of a cementless Ti6Al4V femoral stem (Fig. 1) [3]. This paper anal-yses whether this stem model would pass the endurance test according to ISO 7206 standard. Because the test

Fig. 1. Photograph of broken prosthesis (the arrow indicates the fracture initiation point). Remnants of bone tissue may be seen in thestem proximal area, indicating good osseointegration.

J. Chao / Engineering Failure Analysis 15 (2008) 83–89 85

load specified in ISO standard seems, on the basis of previous works, unsuitable to test the endurance of stemsto be implanted in patients of 1000 N body weight (BW), it is also considered an applied load of 4000 Ninstead of the 2300 N specified in ISO standard.

2. Background

Three patients of about 60 years old and a body mass index above 30 kg/m2 underwent THA in differenthospitals with different surgeons. A cementless Ti6Al4V femoral stem of the same model was used for allpatients. The stem was coated in the proximal third with a beaded porous coating of Ti6Al4V. Fracturesoccurred in vivo suddenly after about four years of implantation without any preliminary pain. The threepatients underwent a revision surgery with replacement of the fractured stems, during which great difficultywas experienced in removing the well fixed fragment of the broken stems. The proximal third of the stemswas covered with remains of bony tissue, confirming osseointegration of the stem by bony ingrowth(Fig. 1). Fortuitous mechanical failure of the implants was ruled out because very similar fractures wereobserved in all cases.

Recently it was shown that fracture of these prostheses was produced by a fatigue process initiated at theneck–shoulder junction [3]. The fracture was initiated in the lateral region of the neck, propagating throughthe cross section in the latero-medial direction, and ended in overload failure of the remaining cross sectionaccompanied with gross plastic deformation. No superficial marks attributable to physical damage duringimplantation or extraction were visible on the neck. No microstructural defects were observed in cross sectionsof the stem near the fracture or in the fracture surface.

3. Analysis method

The parts 4 and 8 of 7206 ISO standard [10,11] constitute a laboratory procedure to determine theendurance of femoral components of hip prostheses. According to this standard a compression loadbetween 300 and 2300 N has to be applied on the head of the prosthesis at an angle of 10� in abductionand 9� in flexion to the stem axis. Since the hip prostheses here analysed were manufactured before the year2002, subsequent modifications to ISO standard have not been considered. Therefore, the stem was con-strained in a distance of 80 mm below the centre of the head of the prosthesis. In accordance with previouswork, the experimental constraint of the stem defined by the ISO standard was simulated with a couple ofbearing points on the frontal and sagittal planes located at 80 and 160 mm from the centre of the femoralhead and fixing the distal tip against rotation [12]. The maximum stresses on the cross section where thebreak occurred (A–A 0 section in Fig. 2) and the corresponding to 80 mm from the centre of the head(B–B 0 section in Fig. 2) were calculated according to the methods used in the material strength handbooks,e.g. [12]. It was assumed that the displacements upon loading are small. From the analysis of the literature,the hip joint forces obtained using telemetric hip prostheses ranged from 2.8 to 5.2 times the BW, depend-ing of the kind of physical activities [13–16]. Assuming an average value of 4 times the BW, the specifiedmaximum load in ISO standard (2300 N) should be adequate to a BW of about 575 N, which is accordancewith previous theoretical predictions [17]. Evidently for BW here considered of about 1000 N it seems moresuitable to apply a test load of 4000 N instead of that of 2300 N specified by ISO standard irrespective ofthe patient BW. A load of 2300 N was chosen to calculate the maximum equivalent stress at the abovementioned cross sections.

Fig. 2 presents a sketch of the stem showing the dimensions and parameters necessary for the calculation ofstresses at A–A 0 and B–B 0 sections according to the following formulation.

Section A–A 0. The hip force F was resolved into F1(=Fcos9�) and F2 (=Fsin9�) components: the former wasobtained from the projection of F on the plane defined by the neck and distal axes of the stem, whereas F2

component is normal to this plane. The component F1 is further resolved into the parallel F3(=F1cos55�)and normal F4 (=F1sin55�) components to the A–A 0 section. The normal load component (F4) gives rise toa compression stress state. The in-plane components (F2 and F3) give rise to My = F2d and Mx = F3d bendingmoments and to the corresponding shear stresses.

Fig. 2. Scheme showing both the geometrical details of the femoral component and the arrangements for fatigue test according to parts 4and 8 of 7206 ISO Standard. The dimensions and the values of the parameters necessary to calculate the stresses at AA’ and BB’ crosssections are also shown. The hollow arrows in the AA’ and BB’ cross sections indicate the location of the maximum equivalent stress(a) medial view and (b) posterior view.

86 J. Chao / Engineering Failure Analysis 15 (2008) 83–89

The net axial stress in a generic (x,y) point of the cross section is given by:

rz ¼4F 4

pD2þMx

Ixy�My

Iyx ð1Þ

In this equation, the inertia moments (Ix and Iy) are given by:

Ix ¼ Iy ¼pD4

64ð2Þ

in which D is the neck diameter.The shear stresses in a (x,y) generic point are given by:

szx ¼ �QyF 2

IyDand szy ¼

QxF 3

IxDð3Þ

in which

Qx ¼1

12ðD2 � 4y2Þ

32 and Qy ¼

1

12ðD2 � 4x2Þ

32 ð4Þ

are the first order moments of the area.

J. Chao / Engineering Failure Analysis 15 (2008) 83–89 87

The Von Mises criterion was used to calculate the equivalent stress corresponding to the combination of theabove stresses, which is expressed by:

TableMaximof max

Cross

A–A 0

B–B0

req ¼ ½r2z þ 3ðs2

zx þ s2zyÞ�

1=2 ð5Þ

Section B–B 0. The hip force F was resolved into F1 and F2 components as in the case of A–A 0 section. However,F1 component was resolved into components normal F5( = F1cos10�) and parallel F6(=F1sin10�) to B–B 0 plane.The load component F5 gives rise to a compression stress state and a pure Mx=F5d1 bending moment. The com-ponents F2 and F6 produce My = F2d2 and Mx = F6d2 bending moments, and the corresponding shear stresses.

The expression of the net axial stress (rz) is that of Eq. (1) but with the corresponding values of the loadcomponents, cross section area and inertia moments to B–B 0 section. The expressions of the inertia momentsof this particular cross section are given by [18]:

Ix ¼ 0:25b 0:055b3 þ 0:7854bð0:17977b2 þ 0:848bhþ h2Þ þ h3

3

� �ð6Þ

Iy ¼b3ð16hþ 3pbÞ

192ð7Þ

The shear stresses for a (x,y) generic point are given by:

szx ¼ �QyF 2

bIyand szy ¼

QxF 6

ðbþ hÞIxð8Þ

in which Qx was obtained by numerical integration and Qy is given by:

Qy ¼ ðb2 � 4x2Þ h8þ 1

3ðb2 � 4x2Þ

12

� �ð9Þ

In addition, F2 component produces a T = F2d1 torsion moment which induces for a generic (x,y) point thefollowing shear stresses:

szx ¼ �T

2Ixy; szy ¼

T2Iy

x ð10Þ

The equivalent stress is given by Eq. (5).The obtained equivalent stress was compared with the endurance limit to assess whether or not this stress

could cause fatigue fracture. The endurance limit of the prosthesis neck (A–A 0 section) was estimated as the ablestress to withstand 5 · 106 cycles without failure of a notched specimen with a stress concentration factor(k = 1.95) identical to that introduced at the prosthesis neck by the design of stem. In an identical way the endur-ance limit of the B–B 0 section was estimated on smooth specimens. Such values have previously been obtainedwith specimens machined from bars of Ti6Al4V ELI surgical alloy with a microstructure similar to that of thefractured stem [3]. The tests were performed at room temperature in laboratory air under tension–tension load-ing conditions with a minimum-to-maximum stress ratio of 0.1. The frequency used in these tests was 20 Hz.

4. Results and discussion

Table 1 shows the values of the maximum equivalent stress normalised by the endurance limit in the A–A 0

and B–B 0 sections. In both cross sections the maximum stress is located in the antero-lateral region of the cross

1um equivalent stresses, requiv; coordinates (x,y) of the location of maximum equivalent stress; endurance limit, rfatigue; and the ratioimum equivalent stress to endurance limit of AA 0 and BB 0 cross sections

section requiv (MPa) (x,y) (mm) rfatigue (MPa) requiv/rfatigue

184 (0.6, �6.5) 210 0.876206 (3.4,�6.9) 536 0.384

88 J. Chao / Engineering Failure Analysis 15 (2008) 83–89

section. The normalisation procedure used involves that the fracture of the prosthesis occurs when the ratio ishigher than or equal to one.

The results show that the prosthesis fulfils the endurance test in accordance with parts 4 and 8 of ISO stan-dard 7206 since the value of the equivalent stress to fatigue strength ratio for 2300 N load is less than 1. How-ever, the value of 0.876 obtained at the neck is far from being conservative, especially when it is consideredthat the fatigue strength was overestimated since the tests were performed in the laboratory atmosphere ata frequency of 20 Hz and not in the more realistic conditions of a frequency of 1 Hz at a saline environment.Moreover, if the angle of flexion of the stem were supposed to be 3� instead of the 10� specified by the stan-dard, the above calculations would predict the failure of the prosthesis at the neck.

The fracture occurred in patients with a BW of about 1000 N. Having into account that the hip joint forceis about 4 times BW, the maximum equivalent stress is 1.74 times higher than that of 2300 N. Thus, failure atthe neck of the femoral component is predicted even when their activity was limited to slow walking.

To prevent stem fracture it is necessary to increase the neck cross section and the radius of the neck–shoul-der junction in order to decrease both the stress and the stress concentration factor at the neck. These mod-ifications would not affect either the flexibility of the stem and in turn to the osseointegration of the implant.

5. Conclusions

� The prostheses can fulfil the requirements specified in parts 4 and 8 of ISO standard 7206.� The ISO standard 7206 is found to be unsuitable for patients of a weight of nearly 1000 N.� It is necessary to consider the patient weight in order to properly design and to choose the more appropriate

stem.

Acknowledgements

The author likes to thank Ms. Carmen Pena for the assistance with the prosthesis drawing and Dr. J.L.Gonzalez-Carrasco for reviewing the manuscript.

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